Many automatic sliding doors are equipped with safety systems intended to detect potential interference with the closing operation of the doors. These safety systems usually include a plurality of signal sources disposed on one door and a plurality of receivers disposed on the other door. The signal sources emit a curtain of signals across the threshold of the door to be detected by the plurality of receivers. When the signal curtain is interrupted, the safety system communicates with a door controller either to cease closing operation and open the doors or to maintain the doors open, depending on the initial position of the doors.
A doorway safety system described in U.S. Pat. No. 4,029,176 to Gerald W. Mills and entitled "Doorway Safety Device" uses acoustic wave transmitters and receivers to detect endangered objects or persons. Not only does the patented system detect objects positioned between the doors and across the threshold, but it also extends the zone of detection into the entryway. The transmitters send out a signal at an angle into the entryway. When an obstruction enters the detection zone, the signal reflects from the obstruction and is detected by the receivers.
Similarly, a published European Patent Application No. EP 0699619A2 to Memco Limited and entitled "Lift Installation for Preventing Premature Closure of the Sliding Doors" describes a three-dimensional system for detecting objects or persons not only across the threshold, but also in the entryway.
One shortcoming of the existing safety systems is detection of objects after the doors have been partially closed. As the doors are closing, the detection zone is also moving and structural obstructions, such as the walls supporting the doors or an outside set of doors, fall within the detection zone. Once the signal is intercepted by a structural obstruction, it is reflected to another structural obstruction and is subsequently detected by the receivers. As the doors are closing and the distance between the transmitters and receivers becomes progressively smaller, the signal that is reflected from the walls and other architectural obstructions travels shorter distances and still remains strong when received by the receivers. The existing safety systems are not able to discriminate between the signal that is reflected from false targets at relatively short distances between the doors and a signal reflected from a true obstruction. The strong signal overloads the receivers. Thus, as the doors close, the safety systems lose the ability to function properly. Many existing safety systems are turned off at some point during closure to avoid false target detections.
The European patent application described above attempts to solve the problem by reducing the gain of the receivers. However, the downside of reducing the gain in the receivers is that actual targets are also not detected.